SKY. GEOLOGY. TRAP VEINS. 397 



being rendered perfect by the channelled marks of the 

 strata formerly described. These veins are often stratified, 

 or more properly speaking, laminated in the direction of 

 their length. They are generally formed of a blueish black 

 basalt : at times they are porphyritic, or vary in other ways 

 which it is unnecessary to describe. I observed in one of 

 them nodules of prehnite, the only occasion on which 

 I have found that mineral in veins of trap. In another 

 a second vein is seen, holding a serpentine course 

 through the first in a somewhat parallel direction, and 

 readily distinguished by being formed of a much more 

 black, durable and compact basalt : it is represented in 

 the accompanying sketch.* 



The peculiar circumstances and extraordinary number of 

 these veins induced me to follow them where they should 

 be expected to reappear. As a preliminary to this attempt 

 it would have been desirable to find their true bearings ; 

 but the want of parallelism in their courses, and the devia- 

 tions from a rectilinear direction, rendered this attempt 

 abortive. They appear however to possess an average com- 

 mon bearing, crossing the line of the coast obliquely, with 

 a general tendency to the northward of west. In at- 

 tempting to trace them in this direction they disappear 

 in the superincumbent trap, none being found on the 

 western side of the promontory. Hence it is probable 

 that they are all branches from that mass, a circumstance 

 in some measure confirmed by the appearances at 

 Swishnish point formerly described. On examining the 

 opposed shore of Sleat, in which trap veins also occur, 

 they are found sparingly dispersed towards the point 

 of that promontory, and still more rarely to the northward 

 of Ord. But at an intermediate part of this coast, and 

 principally in the vicinity of Tarskavig, they are very 

 numerous, and nearly as frequent for a short space as 

 on the shore of Strathaird. Whether or not these are 



* Plate XVIII. fig. i. 



